Rare earth elements are a set of fifteen chemical elements in the periodic table, consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). While named rare earths, they are in fact not that rare and are relatively abundant in the Earth's crust with the exception of promethium, which has no stable nuclear isotope.
The rare earth nitrides (RENs) form in the face-centered cubic (FCC) rocksalt NaCl structure with lattice constants ranging from ˜5.3 Å for LaN to ˜4.76 Å for LuN, in total a 5% difference across the series and less than 0.5% between nitrides of neighbouring atomic species. There is clearly potential for epitaxial growth of custom-designed heterostructures, including superlattices, and even for controlled strains to be introduced.
Most of the fifteen RENs are intrinsic ferromagnetic semiconductors with magnetic properties that provide interesting contrasts and promising complementary electronic properties that make them genuinely attractive for spintronics applications. The RENs exhibit a wide variety of hard- and soft-ferromagnetic properties, i.e. the series includes members with small and huge coercive fields. The best example is GdN and SmN; GdN has a coercive field as small as ˜0.01 Tesla, while in contrast SmN has a coercive field in excess of 6 Tesla.
One of the difficulties which has hindered REN epitaxial growth is the lack of native substrates. A plethora of suitable material that is more or less lattice matched with the RENs has been employed with most research having selected (001) oriented substrate surfaces for REN epitaxy. The resulting REN epitaxial film is fully (001)-oriented, i.e. the (001) direction of the FCC structure of the REN being perpendicular to the surface of the substrate.
The most preferable substrate for REN epitaxy is Yttria-Stabilized Zirconia (YSZ) (001) as it takes an FCC structure, similar to the RENs, with a lattice parameter of 5.125 Å, nearly matching all the REN series.
Historically, the first epitaxial growths of RENs, CeN and then GdN, have been performed on a MgO (001) substrate; but at this time it has been observed that thick films could lead to very rough surfaces.
There are currently broad investigations on combining the rare-earth nitride with group III-nitrides (GaN, AlN, InN) and its alloys, a technologically important non-magnetic semiconductor family used for the fabrication of white and blue light emitting diodes (LEDs) and high power high frequency electronics. This approach aims at taking the best characteristics of both materials and thus lays the foundation for the development of a sustainable technological platform for semiconductor-based spintronics and electronics. Proof of concept devices by combining both materials have already been obtained such as a field effect transistor and GaN-based light emitting diodes with enhanced efficiency.
Group III-nitrides crystallize in either the cubic zinc blende or hexagonal wurtzite structure. Under ambient conditions, the thermodynamically stable structure is the hexagonal wurtzite structure, and commercially available devices such as blue and white LEDs have also a hexagonal wurtzite structure. The wurtzite crystal structure is a member of the hexagonal crystal system or family. Its space group is P63mc in Hermann-Mauguin notation or No. 186 (in the International Union of Crystallography classification).
When REN thin films are epitaxially grown on a hexagonal (0001) wurtzite surface of a group-III nitride, the resulting REN epitaxial film is fully (111)-oriented, i.e. the (111) direction of the FCC structure of the REN layer is perpendicular to the surface of the substrate.
It is not currently possible to grow fully (001) epitaxial layers of RENs on a hexagonal (0001) wurtzite surface of group-III nitrides.
In fact, it is generally extremely rare in materials science to grow a fully oriented (001) rocksalt epilayer on a hexagonal substrate or surface. The extreme difficulty in preparing a fully (001) oriented domain arises from epitaxial relationships and energetic considerations between the epilayer and the substrate that favor the growth in a preferential (111) orientation, where each (111) plane contains one kind of atoms only disposed in a hexagonal mesh.